This a really easy issue to fix for signal.
Signal could enable it on a per-user basis. Why would your friends keep using signal? Because you would not enable it so they won’t be able to talk to you. However, they can now enable it and keep talking to everyone else, so they might decide to delete WhatsApp with in time.
Would they? Signal could make this a choice per user. As in, you as a signal user don’t enable it so they can’t msg you, but they can enable it from signal to talk to you and their social circle all at once.